Battery cell with improved pressure relief vent

ABSTRACT

An electrochemical cell with a pressure relief vent formed in a metal plate disposed in at least one of the closed end and the open end of the container. The pressure relief vent has an annular ring that includes a reduced thickness groove, interrupted in at least two places by unthinned sections of the plate. When the vent opens to relieve pressure from within the cell, the area of the plate within the annular ring remains attached to the remainder of the plate by at least one of the unthinned sections. A can with such a pressure relief vent formed in the can bottom is useful as an electrochemical cell container.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser No.10/365,197, filed Feb. 11, 2003, entitled Battery Cell with ImprovedPressure Relief Vent, currently pending.

BACKGROUND

The present invention generally relates to an electrochemical batterycell. More particularly, the present invention relates to battery cellswith improved pressure relief vents.

Increasing the discharge capacity of electrochemical cells is an ongoingobjective of manufacturers of electrochemical cells and batteries. Oftenthere are certain maximum external dimensions that place constraints onthe volume of a given type of cell or battery. These maximum dimensionsmay be imposed through industry standards or by the amount of spaceavailable into which the cells or batteries can be put. These dimensionslimit maximum cell and battery volumes. Only a portion of the volume isavailable for the materials (electrochemically active materials andelectrolyte) necessary for the electrochemical discharge reactions,because other essential, but inert, components (e.g., containers, seals,terminals, current collectors, and separators) also take up volume. Acertain amount of void volume may also be necessary inside the cells toaccommodate reaction products and increases in material volumes due toother factors, such as high temperature. To maximize discharge capacityin a cell or battery with a limited volume, it is desirable to minimizethe volumes of inert components.

Electrochemical cells are capable of generating gas, during storage,during normal operation, and, especially, under common abusiveconditions, such as forced deep discharging and, for primary cells,charging. Cells are designed to release internal pressure in acontrolled manner. A common approach is to provide a pressure reliefmechanism, or vent, which releases gases from the cell when the internalpressure exceeds a predetermined level. Pressure relief vents often takeup additional internal volume because clearance is generally neededbetween the vent and other cell or battery components in order to insureproper mechanical operation of the mechanism.

Dimensions of consumer cylindrical alkaline batteries are specified inan international standard (International Electrical CommissionPublication 60086-2). Such cells have a positive electrode containingmanganese dioxide, a negative electrode containing zinc, and an alkalineaqueous electrolyte typically containing potassium hydroxide. They oftenhave a cylindrical steel can that serves as the cell container, with thepositive electrode (cathode) formed in a hollow cylindrical shapeagainst the interior surface of the can. A gelled negative electrode(anode) is centrally disposed within the cylindrical cavity in thecathode. An ion-permeable, electrically insulating separator is placedbetween the anode and adjacent surfaces of both the cathode and thebottom of the can. Electrolyte solution is contained within both theanode and the cathode. The can, which is in direct contact with thecathode, serves as the cathode current collector. The open top portionof the can is closed with a closing element, typically including anannular polymeric seal. An outer cover is generally placed over the sealto serve as a negative terminal for the cell. In addition to closing thecan, the seal also electrically insulates the negative terminal from thecan. An anode current collector, usually in the form of a brass nail orwire, extends through an aperture in the center of the seal and into theanode within the cell. The end of the anode current collector on theoutside of the cell makes electrical contact with the negative terminal.The bottom of the can may be flat, or it may be formed to have a centralprotruding nubbin that serves as the positive terminal of the cell. Ifthe can bottom is flat, a separate metal cover is normally affixed tothe can bottom as the positive terminal. A jacket, often an electricallyinsulating, adhesive film label, is generally placed around the sidewalls of the can. Cells may include additional features. For example, aninner cover or a bushing may be disposed between the seal and thenegative terminal to provide a rigid member for maintaining acompressive seal between the seal and the surface of the can and/oranode current collector. In such cells the seal also typically containsa pressure relief vent. This feature usually includes a thinned area,which is designed to rupture when the internal pressure goes above apredetermined level. Examples of cells with seal designs of this typecan be found in U.S. Pat. Nos. 5,227,261 and 6,312,850. However, thistype of seal requires a relatively large amount of volume in order forthe pressure relief vent to function as intended.

In order to increase the amount of active materials in cylindricalalkaline cells, more volume efficient cell designs have been developed.In some of these, the pressure relief vent has been taken out of theseal and put into either a metal cover outside the seal or into thebottom of the can. Many different designs are possible for pressurerelief vents formed in metal plates, whether covers or can bottoms, forelectrochemical cells. Some of these include raised ridges or depressedtroughs, projecting outward or inward, respectively, from the surface ofthe vent-containing plate. Examples of cells with such pressure reliefvent designs can be found in U.S. Pat. Nos. 3,831,822; 3,918,610;4,484,691; 4,601,959; 4,789,608; 5,042,675; and 5,197,622. Each of thesereferences suffers from one or more disadvantages. For example, they mayrely on deformation of the plate at the ridges/troughs to concentratestress in a weakened portion of the plate for the vent to open. This mayrequire a relatively large deflection in the surface of the plate, whichis counter-productive when maximizing the internal volume of the cellfor active materials is an objective. Such designs may also berelatively complicated and difficult to manufacture, which can makeprecise, reliable control of the pressure at which the vent opensdifficult.

Other pressure relief designs do not have ridges or troughs. Some ofthese have grooves, or scores, of reduced thickness in the surface ofthe vent-containing plate. These grooves create weak spots in the platethat are designed to tear or rupture when the pressure differentialbetween the two sides of the plate becomes too great. A variety of suchgrooves can be used. For example, the groove may be in the form of: acircle, a partial circle, one or more curved lines, one or more straightlines, or two or more intersecting straight and/or curved lines. Thegrooves may be formed in the plate in any of a number of possible ways,such as by stamping, coining, scoring, and etching. It may be possibleto combine the step of forming the pressure relief vent for anelectrochemical cell with the process of making the component in whichit is formed. The grooves may be formed when a cover or a can is formed,for example by stamping and/or drawing, using punches and dies, such asin a multiple-stage progressive die set or transfer press tooling. Oneor more steps of such a process can be modified and/or added to includethe formation of the vent grooves.

Information relevant to previous attempts to address the above problemsby using a cell design with a grooved pressure relief vent in a metalcell cover or the bottom of the can may be found in U.S. Pat. Nos.3,074,602; 4,010,044; 4,256,812; 4,698,282; 4,803,136; 4,842,965;6,265,101; 6,303,246; 6,346,342; and 6,348,281. Additional examples maybe found in Japanese unexamined patent publication numbers 01-309,253 A;09-139,197 A; 10-092,397 A; 11-213,978 A; and 11-250,886 A. However,each of these references suffers from one or more of the disadvantagesdescribed below.

Some grooved vent designs are expensive because they are complex andrequire more expensive tooling. Some designs require tooling that ismore difficult to maintain. Others add unnecessarily to the difficultyand cost of manufacturing because the designs are not symmetrical,placing more stresses on the equipment and tooling, and increasing thefrequency and cost of maintenance and replacement. Other grooved ventdesigns may be unsuitable for use in an electrochemical cell because,when the vent operates, a portion of the vent-containing plate may beejected from the cell. Yet other grooved vent designs require too muchclearance for the vent to function, making less internal volumeavailable for active materials, or do not open a large enough area torelieve the internal cell pressure quickly enough to avoid damage orinjury.

For the foregoing reasons, there is a need for a high-capacityelectrochemical battery cell having a reliable, low-volume,cost-effective pressure relief vent.

SUMMARY

One aspect of the invention is an electrochemical battery cell that hasa reliable, low-volume, cost-effective pressure relief vent. The cellcomprises a positive electrode, a negative electrode, and anelectrolyte; a container comprising a side wall, an open end and aclosed end, the closed end comprising a first metal plate; a closingelement, comprising at least one member selected from the groupconsisting of a second metal plate and a seal, disposed in and closingthe open end of the container; and a pressure relief vent, comprising atleast one interrupted annular reduced thickness groove, formed in avent-containing metal plate. The reduced thickness groove is interruptedin at least two places, each by an unthinned section of thevent-containing metal plate, to form at least two reduced thicknessarcs. The vent is capable of opening at the groove, when an internalcell pressure exceeds a predetermined difference above an externalpressure, such that an area of the vent-containing plate radially insidethe groove remains attached to an area of the vent-containing plateradially outside the groove by one or more of the unthinned sectionsinterrupting the groove. The vent-containing metal plate is at least onemember of the group consisting of the first metal plate and the secondmetal plate.

In an embodiment of the invention, the cell comprises a positiveelectrode comprising manganese dioxide, a negative electrode comprisingzinc, and an electrolyte comprising an alkaline aqueous solution; acontainer comprising a side wall, an open end and a closed end, theclosed end comprising a first metal plate; a closing element, comprisingat least one member selected from the group consisting of a second metalplate and a seal, disposed in and closing the open end of the container;and a pressure relief vent, comprising an annular ring formed in thefirst metal plate. The positive electrode is disposed adjacent to theside wall of the container, and the negative electrode is disposed in acavity within the positive electrode. The annular ring comprises areduced thickness groove that is interrupted in at least two places,each by an unthinned section of the first metal plate, to form at leasttwo reduced thickness arcs. The vent is capable of opening at one ormore of the arcs, when an internal cell pressure exceeds a predetermineddifference above an external pressure, such that an area of the firstmetal plate radially inside the annular ring remains attached to an areaof the first metal plate radially outside the annular ring by one ormore of the unthinned sections interrupting the groove.

Another aspect of the invention is a metal can for use as anelectrochemical battery cell container. The can comprises a side wall,an open end, and a closed end, the closed end comprising an integralmetal plate. The metal plate comprises a first surface, a secondsurface, and an interrupted annular reduced thickness groove formed inat least one of the first and second surfaces. The reduced thicknessgroove is interrupted in at least two places, each by an unthinnedsection of the vent-containing metal plate, to form at least two reducedthickness arcs. The plate is capable of opening at one or more of thearcs, when exposed to a pressure differential between the first andsecond surfaces above a predetermined level, such that an area of theplate radially inside the groove remains attached to an area of theplate radially outside the groove by one or more of the unthinnedsections interrupting the groove.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, advantages and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, appended claims,and accompanying drawings, where:

FIG. 1 is a full sectional view showing a cross-section of an embodimentof the battery cell of the invention;

FIG. 2 is a plan view showing the outer surface of a variation of thebottom of the container of the cell of FIG. 1;

FIG. 3 is a partial sectional view showing a cross-section of thecontainer bottom in FIG. 2 taken at III-III;

FIG. 4 is a partial sectional view of the container bottom in FIG. 3after opening of a pressure relief vent;

FIG. 5 is a partial sectional view of a reduced thickness groove in acontainer bottom;

FIG. 6 is a partial sectional view showing a cross-section of a portionof a second embodiment of the battery cell of the invention;

FIG. 7 is a plan view showing dimensions of the bottom of the containerin FIG. 2; and

FIG. 8 is a partial sectional view showing dimensions of the reducedthickness groove and container bottom in FIG. 5.

DESCRIPTION

A battery cell according to the present invention comprises a pressurerelief vent mechanism in a metal plate near either or both of the bottomor top of the cell. The plate or plates containing the pressure reliefvent and the additional space required for the vent to properly openconsume a small amount of the total cell volume, so the internal volumeof the cell that may contain the electrochemically active materials canbe maximized. The pressure relief vent is designed to be reliable, witha small variability in the pressure at which the vent will operate. Itis also designed for ease and economy of manufacture.

An embodiment of a battery cell according to the present invention isshown in FIG. 1. The cell 10 in FIG. 1 is a cylindrical alkaline cell,but the invention may be adapted to other cell shapes andelectrochemical systems. The cell 10 has a positive electrode (cathode)22, comprising manganese dioxide as the active positive material and anelectrically conductive material such as graphite, and a negativeelectrode (anode) 26, comprising zinc particles in a gelled aqueouselectrolyte solution, in a container 12. An ionically conductive,electrically nonconductive separator 24 is disposed between theelectrodes. The cathode 22 is formed in a hollow cylindrical shapeagainst the inner surface of the side wall 18 of the container 12. Theinner surface of the cathode 22 and the bottom 14 of the container 12form a cavity, lined with the separator 24, within which the anode 26 isdisposed.

The cell container 12 in FIG. 1 is a metal can with a metal plateforming a closed end (bottom) 14. The metal plate 14 may be an integralpart of the container 12, such as in a drawn or extruded can, as shownin FIG. 1. Alternatively, the container may be a tube, such as anextruded tube or a seamed tube, with a metal plate affixed thereto.

The open end 16 of the container 12 is closed with a closing element 38.The closing element 38 comprises at least a seal 32, which may be in theform of a polymeric gasket or grommet, or a metal plate (not shown) thatcan cooperate with the container 12 to enclose the internal cellcomponents and keep them sealed inside the cell 10. In the embodimentshown, the closing element 38 also comprises a negative contact terminal30 disposed over the seal 32. In some embodiments, the negative contactterminal 30 may comprise the metal plate of the closing element 38. Theclosing element 38 may comprise one or more additional components, suchas an anode current collector 28, a separate metal plate disposedbetween the negative terminal 30 and the seal 32, and a compressionbushing (not shown). The compression bushing may be used to hold theseal 32 in compression against the anode current collector 28. The anodecurrent collector 28 may be connected to the negative terminal 30 by aweld 34, or the negative terminal 30 may be biased against the anodecurrent collector 28 to maintain electrical contact. The closing elementmay be supported by an annular bead 36 near the top of the side wall 18of the container 12.

The cell 10 in FIG. 1 has a pressure relief vent 40 in the bottom 14 ofthe container 12 to release gas from the cell 10 in a controlled mannerif the internal pressure increases above a predetermined level.Alternatively, a pressure relief vent may be formed in a metal plate inthe closing element (e.g., in either a negative terminal or a inseparate cover between the negative terminal and the seal), or apressure relief vent may be formed in both the closed end of the can andin the closing element. The invention can also be used in combinationwith a conventional pressure relief vent in the seal, e.g., to provide aredundant vent.

The cell 10 has a positive contact terminal 42 disposed on the bottom 14of the container 12, extending over the vent 40. The positive terminal42 may be affixed with welds 54 or by other means that will keep thepositive terminal 42 in physical and electrical contact with the bottom14 of the container 12. The welds 54 are positioned on the peripheralflange 48 of the positive terminal 42, close to the upstanding wall 46of the nubbin 44 to minimize any constraint by the positive terminal 42on bulging of the bottom 14 of the container 12 when pressure builds upinside the cell 10. The positive terminal 42 has a protruding nubbin 44.The positive terminal 42 not only provides an electrical contactsurface, but it also provides an attractive, corrosion resistant coverover the bottom 14 of the container 12 and protects the vent 40 fromdamage. The nubbin 44 provides an open space into which the vent 40 canopen without interference from another cell or an external electricalcontact. The positive terminal 42 also helps to contain internal cellcomponents when the internal pressure is released through the open vent40. One or more openings 56 may also be provided in positive terminal 42to allow gas to escape.

The cell 10 may be used as a single cell battery or as a component cellin a multiple cell battery. When used as a single cell battery, a jacket20, sometimes in the form of a heat-shrinkable adhesive label, isdisposed around the side wall 18 of the container 12, with the distaledges of the jacket 20 generally extending over the edges of thecontainer 12.

An embodiment of a pressure relief vent of a cell of the presentinvention is shown in FIG. 2, which is a plan view of the outer surfaceof the bottom 14 of the container 12 of the cell 10 of FIG. 1. Thepressure relief vent 40 is an interrupted annular ring 58, in the formof a broken reduced thickness groove, formed in the metal plate 14comprising the closed bottom end of the cell 10. The unthinned sections62 a and 62 b interrupt the groove 58 to form two reduced thickness arcs60 a and 60 b. Adjacent ends of the arcs 60 a and 60 b are separatedfrom each other by unthinned sections 62 a and 62 b. Annular ring 58defines an area inside the ring 64 and an area outside the ring 66. Thearcs 60 a and 60 b may be formed in either surface of the metal plate14, but forming them in the outer surface provides a smooth surface incontact with internal components of the cell 10. As shown in FIG. 3,which is a cross section of the container bottom 14 in FIG. 2 taken atIII-III, the metal plate 14 has a thickness 68 beyond the reducedthickness arcs 60 a and 60 b. The arcs 60 a and 60 b each comprise agroove having a depth 72, each groove defining a thinned thickness 70 ofmetal plate 14 at the deepest part of the groove.

The pressure relief vent 40 operates to- release gas and/or liquid frominside the cell 10 when the internal pressure of the cell 10 exceeds apredetermined limit. The limit is selected such that the vent 40 willopen before either the can 12 ruptures elsewhere or the negativeterminal 30 or part of the closing element 38 is ejected from the cell10. When the pressure limit is exceeded, the metal plate 14 tears orfractures at one or more of the reduced thickness arcs 60 a and 60 b.The pressure inside the cell 10 is reduced when gas or liquid escapesfrom the resultant opening(s) in the metal plate 14. The metal plate 14in FIG. 3 is shown after the vent 40 opens in FIG. 4. Portions 74 a and74 b of the area inside the annular ring 64 are deflected generallyoutward, away from the cell 10. The area inside the ring 64 remainsattached to the area outside the ring 66 at one or. both of theunthinned sections 62 a and 62 b, creating hinges at which the outwarddeflected portions 74 a and 74 b swing open. To prevent interferencewith the opening of the vent 40, there is sufficient clearance to theoutside of the metal plate 14 for portions 74 a and 74 b to deflectoutward unimpeded. When the area inside the annular ring 64 remainsattached to the area outside the ring 66 at both unthinned sections 62 aand 62 b, the amount of clearance required for unimpeded opening of thevent is reduced, while providing a large opening area in the metal plate14 to reduce the internal cell pressure quickly. If the annular ring 64is interrupted in more than 2 places, less clearance may be required,but the area of the opening may also be less. FIG. 4 shows the vent openat both reduced thickness arcs. Generally the vent will open at only onearc—the weaker one. Therefore, the invention can provide redundantvents. If one arc is unable to operate as intended, the vent can open atthe other arc to release pressure from the cell.

The cell 10 may have a positive terminal cover extending over the vent40. This can provide protection for the vent, insure adequate clearanceto the outside of the vent 40, and provide some containment for materialreleased from inside the cell 10 when the vent 40 opens. The spacebetween the metal plate 14 and the inside of the nubbin 44 of thepositive terminal 42 allows the vent 40 to open outward without theterminal 42 interfering with the outward deflected portions 74 a and 74b.

A positive terminal cover 42 may not be necessary, as long as there issufficient clearance on the outside of vent 40 for proper ventoperation. In such cells the area inside the annular ring 64 is retainedby at least one of the unthinned sections 62 a and 62 b.

The arcs 60 a and 60 b shown in FIG. 3 have a stylized shape withstraight vertical walls, a flat horizontal bottom, and a sharp cornerbetween the walls and bottom. In practice, it is difficult to make agroove with such a shape, even with tooling that has straight edges andsharp corners. The side walls and bottom tend not to be straight orflat, and corners within and at the edges of the groove tend to besomewhat rounded. The thinned grooves may have other cross-sectional.shapes than that shown in FIG. 3. For example, they may have a U- or aV-shape, or a trapezoidal shape such as the groove 160 shown in FIG. 5.

While the embodiment shown in FIG. 2 has a pressure relief vent in theclosed end of the cell, a pressure relief vent may be formed, in asimilar fashion, in a metal plate in a closing element in the open endof the container. Providing a cover, such as the negative terminal coverin FIG. 1, over the vent-containing metal plate in the closing elementcan provide the same advantages as described above for the positiveterminal cover when the vent is in the closed end of the container.However, the vent must be able to open sufficiently, withoutinterference from the negative terminal cover to relieve internal cellpressure within the desired time. A pressure relief vent in the closingelement may require an additional cell component compared to the cell inFIG. 1, but in some circumstances forming a vent in a separate metalplate may be easier than forming a vent in a metal plate that is anintegral part of the can (e.g., if the can is particularly small indiameter).

The vent-containing metal plate in the cell shown in FIG. 1 issubstantially flat. When the reduced thickness interrupted annulargroove is incorporated into a raised or depressed annular ring (e.g., aridge or a trough), the volume taken up by the metal plate and vent canbe greater. In addition, when pressure builds up inside the cell, themetal plate may bulge outward to a greater extent before the vent opens,requiring more clearance to the outside of the vent. Therefore, when theannulus containing the interrupted thinned groove does not protrude fromthe surface of the metal plate, more space is generally available foractive materials.

In the embodiments shown in FIGS. 1-4, the pressure relief vent has tworeduced thickness arcs disposed on a single annular ring. Otherembodiments are envisioned. For example, the vent may include additionalreduced thickness grooves. Additional reduced thickness grooves may bedisposed radially inside or outside the annular ring on which thereduced thickness arcs are disposed. They may be straight or curvedgrooves, intersecting or nonintersecting, though the tooling required tomake vents with nonintersecting grooves tends to be simpler, easier tomake, and easier to maintain. In another example, the vent-containingplate may have more than one annular ring, each of which comprises aninterrupted reduced thickness groove with a different radius ofcurvature. Such a vent can be designed to open at different pressures.For instance, the grooves on an inner ring may be designed to open at arelatively low pressure and have a relatively small open area, whilethose on an outer ring may open at a higher internal pressure to createa larger open area.

In FIG. 2 each of the reduced thickness arcs has a single radius ofcurvature; in other words, the radius of curvature of the radialmidpoint of the arc is constant at all points on the arc, and the radiiof curvature of the arcs have a common locus (i.e., the radial midpointsof the arcs are all disposed on the same annulus). In another embodimentthe arcs can be curved arc shapes that do not have single radii ofcurvature. In yet another the arc radii can be different from the radiusof an annular ring on which the arcs lie; in such embodiments the arcslie on a broader annulus than that in FIG. 2. Such a vent design may beparticularly useful for another cell shape, such as an oval, rather thana circular, cylindrical shape, so the arc shapes could match thecross-sectional shape of the side wall of the cell container.

In general, it is desirable that vent designs require as little of thetotal cell volume as practical and not produce ejected parts when thevent opens. A simple design is usually better than a more complex designthat has no additional advantages.

Pressure relief vents of the invention can be made using any suitablemethod or combination of methods for producing reduced thickness groovesin metal plates. Suitable methods include, but are not limited to,stamping, casting, forging, rolling, cutting, grinding, laser scribing,and chemical etching. Stamping methods, such as coining, areparticularly well suited. When the vent is disposed in the bottom of acan, the can may be made by deep drawing or impact extrusion, and thevent may be formed in the can bottom, either as a separate process or aspart of the can manufacturing process using either transfer orprogressive die technology. In an example of a combined process, cansare deep drawn on a press using a progressive die. A strip of metal isfed into the die and is indexed through a series of punches and dies.Each punch and die draws the can deeper and/or forms the can closer toits final shape. One or more of the punch and die sets coins the reducedthickness grooves into the bottom of the can. This is generally donenear the end of the process. In the last step, the formed cans, completewith pressure relief vents in their bottoms, are punched out of thestrip of metal. The thickness of the can side wall and bottom can becontrolled by a combination of selection of the characteristics andthickness of the metal strip and the amount and location of metalstretching (and thinning) during drawing (a result of the die design).One or more separate steps can also be used to flatten or otherwiseshape the can bottom after forming the reduced thickness grooves, sincecoining the grooves tends to distort the can bottom shape. While thisshaping may thin the can bottom, any such thinning is minimal, and theareas of the can bottom outside the thinned grooves are referred toherein as “unthinned”. Combining the coining of the vent with the candrawing process makes the design of the tooling more complex, but iteliminates the need for a separate process, and it can reduce the totalcost and variability in manufacturing.

As disclosed above, the reduced thickness grooves of the pressure reliefvent can be formed by coining, either during the can/metal platemanufacturing process, or as a separate process. In coining, force isapplied to the metal, located between a punch and a die. Either or bothof the punch and die can include projections, which cause the metal toflow into the desired shape.

In designing a pressure relief vent, it is desirable to take intoaccount manufacturing considerations, such as cost and ease of diedesign and fabrication, operating speeds of equipment, stresses onforming equipment, wear and maintenance of tooling (e.g., punches anddies), variability in manufactured articles, and tolerances of specifieddimensions. It may be desirable to make compromises when somemanufacturing considerations are in conflict with others. When coiningprocesses are used, radially symmetrical vent designs contribute tominimizing stresses on the tooling. This tends to reduce the requiredfrequency of maintenance of the tooling, the frequency of toolingreplacement, and the variability in vent dimensions thereby making apositive contribution to product cost, quality, and reliability. It isdesirable, to the extent practical, to incorporate into the vent designshapes for which tooling is easy to fabricate, wear resistant, and easyto maintain. Therefore, simplicity of the vent design is generallydesirable.

In addition to the above manufacturing considerations, the pressurerelief vent design must be able to operate effectively to releaseexcessive pressure from the cell in a controlled, safe manner. A goodvent design will open at the desired pressure differential between theinside and outside of the cell and do so quickly enough and with asufficiently large open area. Desirable characteristics can also includea minimum vent activation pressure that is well beyond the normaloperating pressure of the cell, a maximum vent activation pressure thatis well below the pressure at which an uncontrolled release would occur,and low variability in vent activation pressure. It can also beadvantageous for the vent to open fully, to create the maximum openingarea, in a very short time. It may also be desirable for the vent tooperate in such a way as to minimize cell distortion (e.g., bulging ofthe can) before, during, and after opening.

Many factors can affect the vent activation pressure and the way inwhich the vent opens. These include, but are not limited to, metal typeand characteristics (e.g., hardness, tensile stress, and elongation),unthinned thickness of the vent-containing plate, thickness of thevent-containing plate in the thinned grooves, cross-sectional shapes ofthe grooves, diameter of the vent-containing plate, planar shapes anddimensions of the grooves, locations of the grooves on thevent-containing plate, and widths of the unthinned sections interruptingthe groove arcs.

Computer modeling software using finite element analysis, such as ABAQUS(from Hibbit, Karlsson & Sorensen, Inc., Pawtucket, R.I., USA) and MARCK 7.3 (from MSC.Software, Los Angeles, Calif., USA), can be a usefultool for designing pressure relief vents; it can take such factors intoaccount. For example, the can material and thickness may be selectedbased on other requirements, such as the cell electrochemistry, size,and method of closing and sealing. Those same factors are also importantin determining the desired pressure at which the pressure relief ventshould open. Finite element analysis can then be used to predict thevent activation pressure for a given vent design and to refine thedesign to meet the needs of a particular cell.

The embodiment of the electrochemical battery cell of the inventionshown in FIG. 1 is a cylindrical alkaline Zn/MnO₂ cell with a containercomprising a metal can with a closed bottom end and an open top end. Aclosing element is disposed in the open end of the can to seal theactive materials and electrolyte in the cell. This embodiment isdescribed in further detail below.

The can may be made of any suitable metal. A suitable metal is one thatcan be formed into the desired shape and can be adapted to seal thecontents within the cell. It will be sufficiently stable, in contactwith both the internal components of the cell and the intended externalenvironment, to provide acceptable performance, even after storage forlong periods of time. Since it also functions as the cathode currentcollector, the can will have good electrical conductivity.

Steel is typically used for alkaline Zn/MnO₂ cells. The external surfaceof the steel container may be plated to provide corrosion resistance,high electrical conductivity, and an attractive appearance. The internalsurface of at least that portion of the side wall in contact with thecathode may be coated with a material, such as graphite, to provide goodelectrical contact between the can and the cathode. An example of asuitable material for alkaline Zn/MnO₂ cell cans is a low carbon,aluminum killed, SAE 1006 or equivalent steel substrate comprisingmaximums of 0.08 weight percent carbon, 0.45 weight percent manganese,0.025 weight percent phosphorous and 0.02 weight percent sulfur. Thegrain size of the steel is ASTM 8 to 12. If the substrate is plated withnickel or nickel and cobalt, the material is annealed afterelectroplating to allow diffusion of iron from the substrate to thesurface. The steel strip may have the following mechanical properties:45,000 pound maximum yield strength; 60,000 pound ultimate strength; 25percent minimum elongation in 2 inches (50.8 mm); and 82 maximumRockwell 15T hardness.

To provide the maximum volume for active materials in the cell, the canwill be as thin as possible, as long as it is strong enough to withstandthe forces of cell manufacture, storage, and use. The can side walls andbottom are typically from about 0.005 inch (0.13 mm) to about 0.014 inch(0.36 mm) thick, usually no more than about 0.010 inch (0.25 mm). Atless than 0.005 inch (0.13 mm), the can sides and/or bottom can bulgetoo much at acceptable internal cell pressures. This can cause problemsgetting batteries into and out of battery compartments. If the can ismore than 0.014 inch (0.36 mm) thick, the volume of the cell availablefor active materials may be unnecessarily reduced. The can walls andbottom may be the same thickness or different thicknesses. The can sidewall can have different thicknesses indifferent areas to achieve thestrength where needed but minimize the amount and volume of materialelsewhere.

The cathode is formed in the shape of a hollow circular cylinder againstthe inner surface of the can side wall. A common alkaline Zn/MnO₂ cellcathode comprises a mixture of MnO2 active material and particles ofgraphite, which is used to increase the electrical conductivity of theelectrode. The MnO₂ is often an electrolytic manganese dioxide (EMD).Suitable alkaline cell grade EMD can be obtained from Kerr-McGeeChemical Corp. (Oklahoma City, Okla., USA) and Erachem Comilog, Inc.(Baltimore, Md., USA). Preferably the EMD is a high-potential EMD(pH-voltage of at least 0.86 volt) with a potassium content less than200 ppm, as disclosed in International Patent Publication No. WO01/11703 Al, published 15 Feb. 2001. The graphite may be an alkalinegrade graphite powder, an expanded graphite, or a mixture thereof. Asuitable expanded graphite, according to International PatentPublication No. WO 99/00270, published 6 Jan. 1999, is available fromSuperior Graphite Co. (Chicago, Ill., USA). The mixture typically alsocomprises water (with or without electrolyte salt), and may also includesmall (typically less than 2 percent by weight) amounts of othermaterials, generally to improve performance in some way. Examples ofsuch performance-enhancing materials include niobium-doped TiO₂, asdisclosed in International Patent Application No. WO 00/79622 A1, andbarium sulfate.

In some cells a binder is added to the cathode mixture to strengthen thecathode. The binder may also have some additional desirable properties.For example, the binder may fiction as a lubricant when the cathode isformed or may retain electrolyte in the cell, facilitating ion mobilityduring discharge., In general, a minimal amount of binder (or none) isused in order to maximize the amounts of active and electricallyconductive materials. When a binder is used it generally comprises about0.1 to 6, more typically 0.2 to 2, weight percent of the solidcomponents of the positive electrode mixture. Suitable binders foralkaline Zn/MnO₂ cathodes include monomers and polymers of materialssuch as acrylic acid, acrylic acid salts, tetrafluoroethylene, calciumstearate, acrylic acid/sodium sulfonate copolymer, and copolymers ofstyrene and one or more of butadiene, isoprene, ethylene butylene, andethylene propylene. Binder materials may be used alone or incombination. CARBOPOL® 940 (an acrylic acid in the 100% acid form fromB. F. Goodrich), Coathylene HA 1681 (a polyethylene from HoechstCelanese), KRATON® G1702 (a diblock copolymer of styrene, ethylene, andpropylene from Kraton Polymers Business), poly (acrylic acid-co-sodium4-styrene sulfonate) have been found to provide good electrode strength.Mixed binders, such as a mixture of CARBOPOL® 940 and either TEFLON®T30B or TEFLON® 6C (tetrafluoroethylenes from E. I. du Pont de Nemours &Co.), can be advantageous. When a mixture of these two materials isused, a CARBOPOL® to TEFLON® weight ratio of from 1:4 to 4:1 can beadvantageous. In general, within this range, the higher the ratio, thestronger the cathode. For example, the cathode is stronger with aCARBOPOL® to TEFLON® weight ratio of 3:1 than with a ratio of 1:1 or1:3. When a CARBOPOL®/TEFLON® mixture is used, the binder level in thecathode may be about 0.2 to 2, preferably 0.2 to 1, weight percent,based on the solid, undissolved components in the cathode mixture.

The amount of water in the mixture is generally from about 1.5 to 8.0percent, based on the weight of the solid, undissolved ingredients inthe cathode prior to molding. A typical range for use in making impactmolded cathodes is 6 to 8 percent. A typical range for use in ringmolding is 1.5 to 6 percent.

Two common methods of forming alkaline cell cathodes are ring moldingand impact molding. In ring molding one or more (usually 3 to 5) ringsare formed and then inserted into the can in a stack (one ring on top ofanother). Good physical and electrical contact between the can and thecathode are desirable. To achieve this the outside diameter of the ringsmay be made slightly larger than the inside diameter of the can toproduce an interference fit, or the rings may be slightly smaller thanthe can to facilitate insertion, after which the rings are reformedslightly by applying force to the inside and/or top surface, therebyforcing cathode mixture firmly against the can. In impact molding thedesired quantity of cathode mixture is put into the bottom of the canand molded to the desired dimensions using a ram that is inserted intothe center of the can.

The percent solids packing of the molded cathode mixture, determined bydividing the sum of (weight/real density) of solid components by theactual volume of the formed cathode, is typically about 70 to 79percent, with 72 percent being most typical in impact molded cathodesand 75-79 percent being most typical in ring molded cathodes.

A separator is inserted into the cavity formed in the cathode toseparate the anode from both the cathode and the can bottom. Theseparator is typically one or two layers of a porous, wet-laid materialof nonwoven synthetic fibers, such as 0.004 inch (0.10 mm) thick VLZ 105grade separator from Nippon Kodoshi Corp. of Kochi-ken, Japan or 0.003inch (0.08 mm) thick grade FS2100/063 separator from FreudenbergVliesstoffe KG of Neuenburg, Germany. Each layer of separator can befolded and preformed into an elongated basket shape from a long strip ofseparator material. The separator covers the entire inside surface ofthe molded cathode and any exposed inside surface of the can bottom andextends upward beyond the top of the molded cathode, often high enoughto contact the inside surface of the closing element when the cell iscompleted.

The anode mixture is typically a flowable gel that is dispensed into thecavity in the cathode and separator. The gel comprises a mixture ofgelled zinc particles. The zinc may be in powder or flake form, or acombination of the two. An unamalgamated zinc alloy comprising bismuth,indium, and aluminum may be used. Zinc powder, preferably having a d₅₀of about 110 μm, may be obtained from Umicore (Brussels, Belgium), andzinc flake (e.g., grade 5454.3) may be obtained from Transmet Corp.(Columbus, Ohio, USA). The anode also comprises water, potassiumhydroxide electrolyte, and a gelling agent. Acrylic acid in the 100%acid form, such as CARBOPOL® 940 from B. F. Goodrich Specialty Chemicals(Cleveland, Ohio, USA) is a common gelling agent. Small amounts of othermaterials may also be added to the anode mixture and/or electrolyte tominimize gas generation in the cell and/or enhance dischargeperformance. Examples of such materials include In(OH)₃, ZnO, and sodiumsilicate.

The total KOH concentration in the electrolyte in the completed cell,including the anode, the cathode, and any additional electrolyte orwater added to the cell, will generally be from about 36 to about 40weight percent.

The relative amounts of cathode and, anode in the cell are balanced sothat if the cell were completely discharged there would be a smallamount of one of the anode and cathode remaining. A slight excess ofanode is often desirable. For example, the nominal ratio of anode tocathode, based on theoretical input capacities of each (assuming a 1.33electron discharge of the MnO₂) may be between 0.90:1 and 0.99:1.

After the anode is dispensed into the cell, a closing element isinserted into the open end of the can. The closing element can includean anode current collector. The current collector extends through anopening in the seal to make contact with the negative terminal cover.The anode collector can be in the form of a nail or pin. The collectorcan be made of brass, coated with a high hydrogen overvoltage material,such as indium or tin. Indium may be applied to the collector byburnishing, as disclosed in U.S. Pat. No. 5,188,869. Alternatively, tinmay be applied by plating.

The cell in FIG. 1 has a pressure relief vent formed in the can bottomto allow the use of a low volume closing element that, does not includea pressure relief vent. Alternatively, because the pressure relief ventof the invention is a low volume vent, it can be formed in a metal platethat is part of a low volume closing element. The closing element inFIG. 1 has a metal cover, which also serves as the negative contactterminal of the cell, and an annular gasket between the cover and thecan. The cover is electrically conductive and can be made of a metal,such as steel. The cover is often plated with nickel on the outersurface to make it corrosion resistant. The cover can also be coated onthe inside surface to prevent gas-producing reactions if electrolyteand/or electrode materials come in contact with it. The gasket can be anelastomeric material capable of creating a compressive seal between thecan and the cover. Suitable gasket materials include nylon,polyethylene, polypropylene, polytetrafluoroethylene, blends of polymers(e.g., polypropylene and an impact modified poly(phenylene oxide) suchas NORYL® EXTEND™ PPX7110 and PPX7125, from General Electric Co. ofPittsfield, Mass., USA), and other polymeric materials with relativelylow cold-flow rates under compression. A sealant may also be used at theinterface between the gasket and can or between the gasket and cover.When the cell is closed, the diameter of the open end of the can isreduced and the top edge of the can is crimped over, pushing the closingelement downward against the bead. Therefore, the can side wall has anannular bead just below the gasket to support the closing member.

Other closing element designs and cell closing processes may be used incells of the invention. An example of a cell 110 in which the cover iscrimped over the outside of the open top of the can is shown in FIG. 6.In this design, the top edge of the can 112 is curled outward forstrength. The open end of the can, beneath the curled edge, is neckedinward so the cover 130 does not extend radially outward farther thanthe outside diameter of the main body of the can. The anode currentcollector 128 is electrically connected to the center of the cover. Theperipheral edge of the cover is crimped inward against the gasket so thecover is locked in place under the curled edge of the can. The gasket132 not only forms a compressive seal between the cover arid can, but itextends across the inside surface of the cover and forms acompressive-seal around the current collector, protecting the cover fromthe cell contents. This design is low-in volume and has the additionaladvantage that little or no axial loading is placed on the can duringcell closing. This can eliminate the need for a bead in the can tosupport the closing member and make the use of a can with a thinner sidewall possible. Other low volume designs may be used in cells of theinvention. Examples include the designs disclosed in U.S. Pat. Nos.6,410,186, 6,368,745, 6,312,850; 6,300,006, 6,300,004, 6,294,283,6,287,350, 6,265,096, and 6,251,536, as well as co-pending U.S. patentapplication Ser. No. 10/034,687, which are hereby incorporated byreference.

Embodiments of the inventions are described in further detail in thefollowing examples.

EXAMPLE 1

Cans suitable for use in LR6 type cells, were made with pressure reliefvents in their bottom ends as follows.

Cans were deep drawn from an aluminum-killed, low carbon steel strip(carbon content of approximately 0.04%) having a nominal thickness of0.010 inch (0.254 mm), a Vickers microhardness of approximately 120, anda grain size of ASTM 8 to 12. The strip was plated with nickel on theoutside surface and nickel and cobalt on the inside surface and wasdiffusion annealed. The cans were manufactured on a U.S. Baird multipletransfer press.

A stamping die incorporating replaceable carbide inserts was used tofabricate the vent. A single coining step was used to form the reducedthickness grooves of pressure relief vents into the outside surfaces ofthe can bottoms. The raised portions of the carbide inserts thatphysically impact against can bottoms to form the grooves were polishedto a surface finish of 2 microinches (0.051 μm) or less using a diamondpaste polishing compound. This was followed by a can bottom flatteningstep, to insure a flat bottom. A water-based lubricant was used to cooland lubricate the steel and die during forming. The finished cans werecleaned by an alkaline cleaning solution to remove lubricant and otherresidue.

Nominal dimensions of the desired LR6 cans were those shown in Table 1.TABLE 1 Description Dimension Can height 1.926 in. (48.92 mm) Height totop of can step 1.807 in. (from bottom outside) (45.90 mm) Outsidediameter of can 0.568 in. (above can step) (14.43 mm) Inside diameter ofcan body 0.526 in. (below can step) (13.36 mm) Can bottom corner radiusof 0.025 in. curvature (outside surface) (0.64 mm) Can side wallthickness 0.0103 in. above step (0.26 mm) Can side wall thickness 0.0098in. below step (0.25 mm) Can bottom thickness 0.010 in. (unthinned)(0.25 mm)

The pressure relief vent design selected is shown in detail in FIGS. 7and 8. Each of the reduced thickness arcs has a single radius ofcurvature equal to the radius of curvature of the annular ring. Themaximum internal pressure at which the pressure relief vent would openin completed cells, including terminal covers and jackets, was set at1650 pounds per square inch (psi) (116.0 kg/cm²). The minimum cell ventpressure was set at 1050 psi (73.8 kg/cm²). These limits for individualLR6 cells corresponded to can vent pressures of 800 to 1400 psi (56.2 to98.4 kg/cm²): Using finite element analysis, the desired nominaldimensions of the pressure relief vent were selected to be those inTable 2 to produce a can with an average vent pressure between 1000 and1150 psi (between 70.3 and 80.8 kg/cm²). The shape of the groove wasselected to facilitate manufacturing. Tooling was easily maintained withthe wall angle and the width at the bottom of the groove shown. Thetooling was initially set for a groove thickness of approximately 0.003inch (0.076 mm) and then adjusted until the desired average ventpressure was obtained. Adjusting the groove thickness can also be aconvenient means of maintaining the desired vent pressure during themanufacturing process. It may be desirable to set a minimum groovethickness limit; 0.002 inch (0.051 mm) was selected as the minimumthickness in this example. The width of the unthinned sections betweenthe ends of the reduced thickness arcs was selected to be 0.060 inch(1.52 mm) to insure that the area of the can bottom within the annularring would remain attached to the area radially outside the annular ringwhen the vent opened. TABLE 2 FIG. 7 & 8 Ref. Description Dimension RRadius of curvature of annular ring 0.100 in. (from longitudinal axis ofcan to radial (2.54 mm) midpoint of ring) W Width of unthinned sectionbetween reduced 0.060 in. thickness arcs (between radial midpoints (1.52mm) of arcs) D Depth of groove 0.007 in. (0.18 mm) T Groove thickness0.003 in. (0.076 mm) G Width of groove at bottom 0.004 in. (0.10 mm) AAngle of groove wall (from vertical) 30 deg. L Arc length of reducethickness arc (each arc) 145 deg.

FIGS. 7 and 8 are stylized drawings, showing grooves with regular, flatsurfaces and sharp angles. Actual formed pressure relief vents will havegrooves with surfaces that are somewhat irregular. In measuringdimensions on actual formed vents, averaging can be used to compensatefor irregularities. Potting and cross sectioning can be used to preparecans for measurement of the pressure relief vent dimensions. Othermethods can also be used to produce comparable results. Nondestructivemeasurements of the groove depth can be made using an instrument such asa SmartScope Model ZIP 250 (Optical Gaging Products, Inc., Rochester,N.Y., USA). The thickness of the can at the bottom of the groove (groovethickness) can be calculated by subtracting this groove depthmeasurement from the unthinned bottom thickness, which can be measuredwith a micrometer gauge.

EXAMPLE 2

Cans from Example 1 were tested to evaluate the pressure at which thepressure relief vents would open. Because cans are more easily testedwhen they are empty than after they have been used in cells, acorrelation was first established between the results of cell and canvent pressure. Live cells were tested hydraulically for both convenienceand to closely approximate the way in which pressure builds up in cells.Empty cans were tested using a pneumatic tester as a matter ofconvenience.

Pneumatic testing of empty cans was done using a Fastest Pneumatic CanVent Tester, Model FES0-04, available from Fastest Corporation, St.Paul, Minn., USA. Each can was tested by placing it into a test fixture,sealing the open end of the can against the fixture, and pressurizingthe inside of the can at a selected rate of about 50 lbs./sec. (3.5kg/sec.) until the pressure relief vent opened. The rate ofpressurization was controlled by adjusting a needle valve on thepressure inlet so that the time interval between pressure readings of300 and 800 psi (21.1 and 56.2 kg/cm²) was 10 seconds. The pressurewithin the can was monitored with an electronic gauge meter with a peakhold feature. The vent pressure of a can was the peak pressure readingfrom this gauge after the vent had opened. The can was also observed todetermine if the area inside the vent remained connected to the areaoutside the vent, and if so, whether at one or both of the unthinnedsections between the reduced thickness arcs of the vent. The testfixture was designed specifically for this testing. The test fixturedesign can affect the test results, so the correlation between cell andcan vent pressures will depend on the test fixtures used.

The vent pressures of cans from Example 1 averaged 1090 psi (76.6kg/cm²), with a standard deviation of 72 psi (5.1 kg/cm²). When the cansvented, no ejection of any portion of the can bottom occurred; thesection of the can bottom inside the annular ring remained attached tothe can in all cases. In contrast, when similar cans with only a 0.050inch (1.27 mm) unthinned section between the ends of the arcs weretested, both unthinned sections sometimes broke, and the portion of thecan bottom within the annular ring was ejected from about 20 percent ofthe cans.

During manufacture of 2,000,000 LR6 cans in Example 1, the groovethickness was checked frequently and can vent pressure was testedperiodically. No significant differences attributable to tooling wearwere observed in groove thickness or can vent pressure over the entireperiod of manufacture. Maintenance of the tooling used to coin thepressure relief vent grooves was not required any more frequently thanfor the normal tooling used in forming cans without pressure relief ventgrooves.

EXAMPLE 3

Cans suitable for use in LR03 type cells were made with the nominaldimensions shown in Table 3. The can manufacturing process was asdescribed in Example 1, using 0.010 inch (0.25 mm) thick steel strip.TABLE 3 Description Dimension Can height 1.689 in. (42.90 mm) Height totop of can step 1.569 in. (from bottom outside) (39.86 mm) Outsidediameter of can 0.4115 in. (above can step) (10.45 mm) Inside diameterof can body 0.380 in. (below can step) (9.66 mm) Can bottom cornerradius of 0.020 in. curvature (outside surface) (0.51 mm) Can side wallthickness 0.010 in. above step (0.25 mm) Can side wall thickness 0.008in. below step (0.21 mm) Can bottom thickness 0.0090-0.0105 in.(unthinned) (0.23-0.27 mm)

The selected pressure relief vent design was that shown in FIGS. 7 and8, with the nominal dimensions shown in Table 4. The shape of the groove(depth, wall angle, and width at the bottom) and the width of theunthinned sections between the ends of the arcs were kept the same as inExample 1. The radius of curvature of the annular ring was reduced toproduce a desired nominal can vent pressure of 1800 psi (126.5 kg/cm²).TABLE 4 FIGS. 7 & 8 Ref. Description Dimension R Radius of curvature ofannular ring (from 0.070 in. longitudinal axis of can to radial midpoint(1.78 mm) of ring) W Width of unthinned section between reduced 0.060in. thickness arcs (between radial midpoints (1.52 mm) of arcs) D Depthof groove 0.007 in. (0.18 mm) T Groove thickness 0.003 in. (0.076 mm) GWidth of groove at bottom 0.004 in. (0.10 mm) A Angle of groove wall(from vertical) 30 deg. L Arc length of reduce thickness arc (each arc)160 deg.

EXAMPLE 4

Vent pressures of cans from Example 3 were determined in the same manneras in Example 2. The actual can vent pressure averaged 1848 psi (129.9kg/cm²), with a standard deviation of 79 psi (5.6 kg/cm²).

EXAMPLE 5

A pressure relief vent was designed according to FIG. 7 for LR6 cellcans made from 0.008 inch (0.20 mm) thick steel strip. A nominal canvent pressure of 800 to 900 psi (56.2 to 63.3 kg/cm²) was selected. Thenominal can dimensions are shown in Table 5, and the nominal ventdimensions are shown in Table 6. Because the cans were made for use incells with a different closing element design than the cans in Example1, the outside diameter of the top portion of the can is different.Reducing the can bottom thickness from 0.010 inch (0.25 mm), as inExample 1, also reduced the can vent pressure, because the required ventpressure will also be lower, so the same vent dimensions as used in 15Example 1 could be used in Example 5. TABLE 5 Description Dimension Canheight 1.924 in. (48.86 mm) Height to top of can step 1.766 in. (frombottom outside) (44.86 mm) Outside diameter of can 0.582 in. (above canstep) (14.78 mm) Inside diameter of can body 0.530 in. (below can step)(13.46 mm) Can bottom corner radius of 0.025 in. curvature (outsidesurface) (0.64 mm) Can side wall thickness 0.008 in. above step (0.20mm) Can side wall thickness 0.008 in. below step (0.20 mm) Can bottomthickness 0.007-0.009 in. (unthinned) (0.18-0.23 mm)

TABLE 6 FIGS. 7 & 8 Ref. Description Dimension R Radius of curvature ofannular ring (from 0.100 in. longitudinal axis of can to radial midpoint(2.54 mm) of ring) W Width of unthinned section between reduced 0.060in. thickness arcs (between radial midpoints (1.52 mm) of arcs) D Depthof groove 0.007 in. (0.18 mm) T Groove thickness 0.003 in. (0.076 mm) GWidth of groove at bottom 0.004 in. (0.10 mm) A Angle of groove wall(from vertical) 30 deg. L Arc length of reduce thickness arc (each arc)145 deg.

EXAMPLE 6

Cans from Example 5 were vent tested as in Example 2. The vent pressuresaveraged 865 psi (60.8 kg/cm²), with a standard deviation of 31.3 psi(2.2 kg/cm²). When the cans vented, no ejection of any portion of thecan bottom occurred; the section of the can bottom inside the annularring remained attached to the can in all cases.

EXAMPLE 7

A pressure relief vent was designed for cans to be used in a LR20 typecell. Nominal can dimensions are shown in Table 7, and nominal pressurerelief vent dimensions are shown in Table 8. The shape of the groove(depth, wall angle, and width at the bottom) and the width of theunthinned sections between the ends of the arcs were kept the same as inExample 1. The radius of curvature of the annular ring was increased toproduce a desired nominal can vent pressure of 475 psi (33.4 kg/cm²).TABLE 7 Description Dimension Can height 2.346 in. (59.59 mm) Height tomidpoint of can 2.088 in. step (from bottom outside) (53.04 mm) Outsidediameter of can 1.319 in. (above can step) (33.50 mm) Inside diameter ofcan body 1.277 in. (below can step) (32.44 mm) Can bottom corner radiusof 0.038 in. curvature (outside surface) (0.97 mm) Can side wallthickness 0.0110-0.0125 in. above step (0.28-0.32 mm) Can side wallthickness 0.0090-0.0110 in. below step (0.23-0.28 mm) Can bottomthickness 0.0095-0.0105 in. (unthinned) (0.24-0.27 mm)

The selected pressure relief vent design was that shown in FIG. 7, withthe nominal dimensions shown in Table 8. TABLE 8 FIGS. 7 & 8 Ref.Description Dimension R Radius of curvature of annular ring (from 0.1745in. longitudinal axis of can to radial midpoint (4.43 mm) of ring) WWidth of unthinned section between reduced 0.060 in. thickness arcs(between radial midpoints (1.52 mm) of arcs) D Depth of groove 0.007 in.(0.18 mm) T Groove thickness 0.003 in. (0.076 mm) G Width of groove atbottom 0.004 in. (0.10 mm) A Angle of groove wall (from vertical) 30deg. L Arc length of reduce thickness arc (each arc) 160 deg.

Though the actual average can vent pressures were not exactly at thepreselected nominal values in Examples 2, 4, and 6, they were withinabout 10%, showing that finite element analysis is a useful tool forpressure relief vent design. If necessary, adjustments to the actualaverage vent pressures can be made by changing the radius of curvatureof the reduced thickness grooves. Alternatively, vent pressures can beadjusted by changing one or more other vent design parameters, such asthe width of the unthinned section between the ends of the reducedthickness arcs, the width at the bottom of the groove, and the groovethickness.

From the dimensions of the pressure relief vents in Examples 1, 3, and7, and the results of the testing in Examples 2 and 4, it is clear that,when the can bottom thickness is not changed, the same vent design canbe readily adapted to cells of different sizes. This can be done bychanging the radius of curvature of the annular ring on which thereduced thickness arcs are placed, interpolating or extrapolating from aplot of the annular ring radius of curvature as a function of candiameter. Minor changes can then be made empirically to adjust theaverage can vent pressure to the desired nominal.

As demonstrated above, the invention is useful in cells of varioussizes, such as cells with container outside diameters from about 0.4inch (10 mm) to about 1.4 inch (36 mm), and with pressure relief ventshaving an interrupted annular ring (groove) radius of curvature about0.06 inch (1.5 mm) to about 0.18 inch (4.6 mm). For LR6 type cells, anannular ring radius of curvature of 0.100 inch (2.54 mm) has beendemonstrated, but other radii of curvature, e.g., from about 0.085 inch(2.16 mm) to about 0.115 inch (2.92 mm), and beyond, can be expected tobe suitable, by adjusting other vent design dimensions.

Although the present invention has been described in considerabledetail, with reference to certain preferred versions thereof, otherversions are possible. Therefore, the spirit and scope of the appendedclaims should not be limited to the description of the preferredversions contained herein.

1. An electrochemical battery cell comprising a positive electrode, anegative electrode and an electrolyte; a container comprising a sidewall, an open end and an integral closed end; a closing element,comprising at least one member selected from the group consisting of asecond metal plate and a seal, disposed in and closing the open end ofthe container; and a pressure relief vent, wherein: the positiveelectrode is adjacent to the container side wall, and the negativeelectrode is disposed within a cylindrical cavity in the positiveelectrode; the pressure relief vent comprises an interrupted, reducedthickness groove formed in the closed end of the container; theinterrupted, reduced thickness groove is interrupted in at least twoplaces, each by an unthinned section of the closed end of the container,to form at least two reduced thickness arcs disposed in a single annularring; the closed end of the container is free of intersecting reducedthickness grooves; and the vent is capable of opening at the groove,when an internal cell pressure exceeds a predetermined difference abovean external pressure, such that an area of the vent-containing plateradially inside the groove remains attached to an area of thevent-containing plate radially outside the groove by one or more of theunthinned sections interrupting the groove.
 2. The cell defined in claim1, wherein the pressure relief vent consists of a single interrupted,reduced thickness groove.
 3. The cell defined in claim 2, wherein theannular ring is oval.
 4. The cell defmed in claim 2, wherein the annularring is circular.
 5. The cell defmed in claim 1, wherein the pressurerelief vent comprises first and second interrupted, reduced thicknessgrooves, each interrupted by unthinned sections of the closed end of thecontainer to form at least two reduced thickness arcs disposed in asingle annular ring, and the first groove is disposed radially insidethe second groove.
 6. The cell defined in claim 5, wherein at least oneof the grooves is oval.
 7. The cell defined in claim 5, wherein at leastone of the grooves is circular.
 8. The cell defined in claim 5, whereinthe difference between the internal pressure and external pressure atwhich the pressure relief vent is capable of opening is different at thefirst and second grooves.
 9. The cell defined in claim 8, wherein thepressure relief vent is capable of opening at the first groove when thedifference between the internal and external pressures is smaller thanat the second groove.
 10. An electrochemical battery cell comprising apositive electrode, a negative electrode and an electrolyte; a containercomprising a side wall, an open end and a closed end; a closing element,comprising at least one member selected from the group consisting of asecond metal plate and a seal, disposed in and closing the open end ofthe container; and a pressure relief vent, wherein: the positiveelectrode is adjacent to the container side wall, and the negativeelectrode is disposed within a cylindrical cavity in the positiveelectrode; the pressure relief vent comprises one or more interrupted,reduced thickness groove formed in the closed end of the container; eachinterrupted, reduced thickness groove is interrupted in at least twoplaces by unthinned sections of the closed end of the container to format least two reduced thickness arcs disposed in a single annular ring;the pressure relief vent includes no reduced thickness grooves locatedradially inside the annular rings of the one or more interrupted,reduced thickness grooves, and the annular rings are mutuallynonintersecting; and the vent is capable of opening at the groove, whenan internal cell pressure exceeds a predetermined difference above anexternal pressure, such that an area of the vent-containing plateradially inside the groove remains attached to an area of thevent-containing plate radially outside the groove by one or more of theunthinned sections interrupting the groove.
 11. The cell defined inclaim 10, wherein one or more of the annular rings are oval.
 12. Thecell defined in claim 10, wherein one or more of the annular rings arecircular.
 13. The cell defined in claim 10, wherein the pressure reliefvent comprises two interrupted, reduced thickness vents, one disposed ona radially inner annular ring, and the other disposed on a radiallyouter annular ring.
 14. The cell defined in claim 10, wherein the cellis a cylindrical cell.
 15. The cell defined in claim 14, wherein thecell is an LR6 alkaline Zn/MnO₂ cell.
 16. The cell defined in claim 14,wherein the cell is an LR03 alkaline Zn/MnO₂ cell.
 17. A metal can foruse as an electrochemical battery cell container, the can comprising aside wall, an open end, and an integral closed end, wherein: the closedend comprises a first surface, a second surface, and a pressure reliefvent; the pressure relief vent consists essentially of one or morenonintersecting, interrupted, annular, reduced thickness grooves, eachformed in at least one of the first and second surfaces; each reducedthickness groove is interrupted in at least two places by two or moreunthinned sections of the vent-containing metal plate to form at leasttwo reduced thickness arcs; and the plate is capable of opening at oneor more of the arcs, when exposed to a pressure differential between thefirst and second surfaces above a predetermined level, such that an areaof the plate radially inside each annular grooves remains attached to anarea of the plate radially outside the one or more annular grooves byone or more of the unthinned sections.
 18. The metal can defined inclaim 17, wherein at least one of the annular grooves is oval.
 19. Themetal can defined in claim 17, wherein at least one of the annulargrooves is circular.